A Moon’s Moon

Q. Is it possible for a moon to have a naturally occurring moon of its own?

A. It is possible, scientists say, but no such submoons are known in our solar system, and even in a faraway system, conditions of mass and distance would have to be just right to allow a moon’s moon to be there.

Even then, an object in orbit around a sizable satellite of a sizable planet would probably meet the same fate that befalls human-made lunar satellites after a certain period: Tidal forces would make its orbit decay, and it would eventually break up or be drawn into the parent moon by the force of gravity.

Scientists think that most of the moons in the solar system were probably formed by the consolidation of disks of dust and gas that circulated around them early in the history of the system. That could happen elsewhere in the universe, or a moon could capture a passing object, a less likely event.

But there is only a limited region in which a lunar satellite could stay in orbit under the influence of a moon’s gravity rather than becoming subject to the gravity of the more massive planet that held that moon. Such a region of gravitational influence is called the Hill sphere, after the American astronomer George William Hill (1838-1914), who first described it.